The present invention relates to a system for maintaining power consumption below a predetermined level and, more particularly, to an add/shed control which utilizes anticipatory processes for maintaining power consumption below a predetermined level where the loads can be shed based upon anticipated future increases in power consumption.
Power utilities charge their customers according to a policy which is designed both to encourage energy conservation and to assess the cost for the extra power generating equipment which they must acquire and maintain to meet peak demands against those creating the peak demand. As a result, these power utilities will typically charge their customers at a first rate for power consumed below a first level, and at a second rate for power consumed between the first level and a second level. If power consumption should exceed the second level, the utilities will charge what isin effect a penalty for the acquisition and maintenance of the extra power generating equipment required to meet these periods of unusually high demand. In addition, many utilities will charge at the higher rates for all power consumed for a period of, for example, a year, if the power demand increases at any time to above the second level.
In order to avoid these peak demand charges imposed upon them by their power utilities, power consumers have employed automatic control systems which monitor power consumption within their facilities and which control the energization state of the power consuming loads within their facilities to maintain power consumption below some predetermined value in order to reduce the extra charges which would otherwise be imposed upon them during peak demand periods or intervals. These systems have typically taken the form of add/shed control systems which are designed to shed loads as power consumption approaches a predetermined level which is established by the customer. Alternatively, as power consumption begins to fall away from this predetermined level, previously shed loads can be added back on-line so that they may be utilized by the customer.
Several different types of add/shed control systems have been utilized in the past. For example, the most common type of add/shed control system established a prioritized load order wherein the load having the lowest priority will be shed first and the load having the highest priority will be shed last. In such a system, if loads can be added back on-line, the load having the highest priority will be added first and the load having the lowest priority will be added last.
Another add/shed system which is known adds and sheds loads on the basis of comfort fairness. That is, the loads associated with zones having conditions closest to set point will have the lowest priority and loads associated with zones having conditions the furthest away from set point will have the highest priority. In this type of system, the loads having lowest priority will be shed first and the loads having the highest priority will be shed last on the basis that it is more fair to shed loads associated with the zones being closest to the comfort range first.
Unfortunately, these systems have been inflexible because rarely can all the loads within a building be made to fit within one of these types of add/shed routines. For example, it is more desirable to shed the air handling fan systems within the building according to a comfort fairness routine than according to a sequential routine because a sequential routine is a fixed routine and will not necessarily result in fair control of the air handling fan systems. On the other hand, the outdoor lights certainly would not be shed on the basis of comfort fairness and it may also be determined that the outdoor lights are all of equal priority so that they could not be prioritized according to a sequential routine. Thus, the outdoor lights may be shed rotationally so that the first shed will be the first added.
As a result, a new and improved add/shed load control was developed by the assignee of the present invention and is fully disclosed in U.S. Pat. No. 4,510,398 which is assigned to the assignee of the present invention. In this system, an add/shed control is provided for maintaining power consumption below a predetermined level where the loads can be added and/or shed in different sequences depending upon the prioritization level given to each load and upon the add/shed sequence assigned to each prioritization level.
While the add/shed control disclosed in the aforementioned U.S. Pat. No. 4,510,398 adds flexibility to such systems, there remains substantial room for improvement. For example, the foregoing system does not provide for determining the anticipated future increase or decrease in power consumption of the building and, as a result, is unable to predict which loads must be shed in order to maintain the power consumption of a building below a predetermined level when future loads are to come on line. Hence, all of the add/shed control systems previously described are reactive systems which can only react to existing circumstances and are unable to maintain power consumption of a building below a predetermined level on a continuous basis based upon known future increases or decreases in power consumption.
In addition to the foregoing, such systems usually include a plurality of loads which are under control of "application packages" which control these loads based upon a predetermined on and off schedule. When one of the loads assigned to an application package is to come on line, the system can only react to the load coming on line which may cause the power consumption of the building to instantaneously exceed the desired predetermined level. It would therefore be advantageous in the art to have a system which could shed loads in anticipation of a load coming on line under control of an application package and to delay the load from coming on line until enough load can be shed to maintain the total power consumption of the building below the predetermined level when the load does in fact come on line.
It is therefore a general object of the present invention to provide a new and improved add/shed control system and method for adding and shedding loads to maintain the power consumption of a building below a predetermined level.
It is another object of the present invention to provide such an add/shed system and method which can determine in advance the future increase or decrease in power consumption of a building and which is capable of shedding enough load to accommodate the future increases in power consumption without exceeding the predetermined power consumption level.
It is a still further object of the present invention to provide such a system and method which can delay the on time of a load until enough load is shed to permit the load to come on line without increasing the power consumption above the predetermined level.